This invention relates to an electric motor unit, more specifically to an electric motor unit for rotating a polygonal mirror.
In general, an optical deflector of a polygonal mirror type is provided with a motor unit to rotate a polygonal mirror at high speed, e.g., scores of thousands of revolutions per minute. In such a motor unit, friction between a motor shaft and bearings need be minimized for high-speed rotation. To meet this requirement, there is proposed a motor unit of a tilting pad type as disclosed in Japanese Patent Publication No. 6854/78. In the motor unit of the tilting pad type, the motor shaft is radially supported by journal bearings of a dynamic pressure type, and is floated or suspended to be axially supported by a repulsive force produced between permanent magnets fixed individually to an end of the motor shaft and a motor housing. Having its motor shaft supported both radially and axially, this motor unit is fit for high-speed rotation, though it has the following drawbacks. Floated or suspended by the repulsive force between the pair of permanent magnets, the motor shaft is liable to vibrate due to external vibration or the like, as well as to become a little eccentric. Moreover, the arrangement of the permanent magnets along the axis of the motor shaft may lead to an increase in size of the motor unit.
The use of the motor unit involving these problems in the optical deflector of the polygonal mirror type will cause the following additional problems. Since the incidence position of a laser beam incident upon the polygonal mirror changes as the motor shaft vibrates along its axis, the width of the polygonal mirror must be great enough. Therefore, the polygonal mirror increases in cost and weight, so that rotatory load on the motor unit increases to reduce the starting capability of the motor unit. In a deflector so designed that the light reflecting surfaces of the polygonal mirror are at an angle to the axis of the motor shaft, the scanning rate of a laser scanned by the polygonal mirror varies as the incidence position of the laser beam is changed by the vibration of the motor shaft.